This invention pertains to the art of nuclear reactor control instrumentation systems and particularly control systems for boiling water reactors.
The invention is more particularly applicable to a wide range flux monitor assembly including an extended range start-up monitor and a power range monitor for monitoring neutron flux over a wide reactor power range. However, it will be appreciated to those skilled in the art that the invention could be readily adapted for use in other environments as, for example, where similar monitoring assemblies are employed to monitor radiation.
Conventional neutron monitoring devices and detection systems have employed a variety of different neutron detectors to cover the entire range of neutron flux levels for light-water-cooled nuclear power plants. For high neutron flux levels, fission chambers comprising direct current operation mode ionization detectors have been utilized, while for low neutron flux levels, a proportional counter detector operated in the pulse mode has been employed.
In response to the accident at Three Mile Island new requirements for instrumentation for light-water-cooled nuclear power plants have been promulgated in Regulatory Guide 1.97 (Rev. 2) of the Nuclear Regulatory Commission. A major objective of these requirements is to ensure the quality and reliability of plant status information available to control room operating personnel both during and after an accident. Thus, in the Guide certain plant variables and systems are identified as essential to the execution of a prompt shutdown and to the verification of the effective operation of plant safety systems. Requirements for instrumentation channels which monitor these key variables are framed to reflect the importance to safety of the measured variables and the need to survive the increased stress of the accident environment. It is imperative that the instrumentation operate in the most severe conditions produced by an accident and for an adequate length of time afterwards. Furthermore, the range of measurement must be wide enough that the instrument is on scale at all times, even under abnormal conditions.
The Guide calls for the measurement of neutron flux as a key variable in monitoring the status of reactivity control. It has been designated as Type B, Category 1, with a range of measurement of 10.sup.-6 % to 100% of full power. In the classification scheme of the guide, a Type B variable is one which provides information to indicate whether plant safety functions are being accomplished. Category 1 denotes the most stringent design and qualification criteria, in accordance with Regulatory Guide 1.89, "Qualification of Class 1E Equipment for Nuclear Power Plants" and Regulatory Guide 1.100, "Seismic Qualification of Electrical Equipment for Nuclear Power Plants."
Since conventional light water reactors are already instrumented with highly developed and sophisticated neutron monitoring systems, it is natural to examine the possibility that they might satisfy the guide requirements presently or with some limited modifications. In fact, the conventional neutron monitoring systems in both Pressurized Water Reactors (PWR) and Boiling Water Reactors (BWR) exceed the range of performance needed. However, the level of qualification for such typical systems does not meet the requirements of Category 1.
In the case of PWR's the trend is to install new, qualified, out of core channels based on high sensitivity fission chambers. For BWR's the leakage neutron flux is quite low outside of the biological shield where ex-core detectors might be located. It is questionable whether detectors placed ex-core could provide the necessary sensitivity to cover the lower flux range. The preferred location for the BWR post-accident neutron flux monitoring detectors is in-core where the normal flux monitoring instrumentation resides.
The flux monitoring instrumentation for a typical BWR cover a range of 10.sup.3 nv to 2.times.10.sup.14 nv using three subsystems:
(1) Retractable Source Range Monitor (SRM) PA1 (2) Retractable Intermediate Range Monitor (IRM) PA1 (3) Fixed Power Range Monitor (PRM)
The detectors in each of these subsystems are miniature fission chambers containing U.sup.235 as the sensitive material. The SRM and the IRM detectors are retracted out of the core during full power operation to minimize burn-up of the sensitive material and prolong life. The rack and pinion drive mechanisms for moving the detectors are located below the reactor vessel. The task of qualifying a complex movable detector system to Category 1 was judged to be a formidable one. In addition, because of the relationship of this instrumentation to the Reactor Protection System, any modifications could entail extensive re-analysis of existing safety systems. These considerations provided an incentive to find another approach to the wide range flux measurement.
Known PRM detectors have employed a regenerative U.sup.234, U.sup.235 coating as the sensitive material to extend the detector life at full power flux. However, the amount of such coating which is applied to a PRM detector for successful operation precludes use of a conventional PRM detector as a reliable SRM or IRM detector. A PRM detector is simply not sensitive enough for source or intermediate range monitoring.
The present invention contemplates a new and improved wide range flux monitor assembly including a fixed in-core extended range start-up monitor for source and intermediate range monitoring and a fixed in-core power range monitor for power range monitoring. The invention overcomes all of the above referred to problems and others to provide a new neutron monitor assembly which may monitor source, intermediate and power range operation of a nuclear reactor while remaining in-core during full range of power operation, is simple in design, economical to manufacture, provides reliable neutron monitoring over a wide range of neutron radiation without a complicated insertion and retraction system and is compatible with pre-existing conventional neutron monitoring systems in BWR reactor assemblies.